Fix an issue when deleting the entire content of an unsubmitted inline comment

Summary: Ref T12733. In this case, the server returns no new row, but we would incorrectly try to bind to one.

Test Plan:

• Created a new comment.
• Edited it.
• Deleted all the text.
• Saved changes.
• Before: Header continues to show phantom "1 Unsubmitted Comment", browser error log reflects one error.
• After: Header reflects comment being deleted, error log is quiet.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D18262